Data is a structure with a reference to an already added key object and
some data that the clients wants signed with that key. If the key object
wasn't loaded, return AGENT_FAILURE, else return the signature.

Use this method to translate to a higher-level message. Usually, some
callback will be made upon the receipt of each complete protocol
message.

Parameters

data

a string of indeterminate length. Please keep in mind that you will
probably need to buffer some data, as partial (or multiple) protocol
messages may be received! I recommend that unit tests for protocols call
through to this method with differing chunk sizes, down to one byte at a
time.

Data is a structure with a reference to an already added key object and
some data that the clients wants signed with that key. If the key object
wasn't loaded, return AGENT_FAILURE, else return the signature.